The present invention is related to protective energy barriers for potentially explosive environments in general, and more particularly to apparatus for and method of protecting the safe side wiring of a protective energy barrier against transferring fault energy into a potentially explosive environment, like an aircraft fuel tank, for example.
Fuel quantity measuring systems (FQMS) are disposed on-board aircraft for determining and monitoring the amount of fuel remaining in an aircraft fuel tank utilizing one or more sensors located within the tank. A block diagram schematic of an exemplary FQMS 10 is shown in FIG. 1. Referring to FIG. 1, the FQMS 10 includes an indicator circuit 12, which is located typically in the cockpit of the aircraft, and a fuel tank 14 which contains one or more sensors (not shown), which may be of the capacitive variety, for example, for sensing the level of the fuel in the tank 14. The indicator circuit 12 typically includes processing circuits and a communication interface to transmit AC excitation signals to each sensor in the tank. Each sensor responds with a response signal, representative of the fuel level, which is fed back to the indicator circuit 12 wherein the response signal is processed to determine the quantity of fuel in the tank.
The sensors within the tank 14 are coupled to the indicator circuit 12 through a plurality of wires 16. In the present example, three wires 16 are used for the capacitive type sensors wherein two wires 16a and 16b are low impedance lines used to couple the excitation signals from the indicator circuit 12 to the sensors, and the third wire 16c is a high impedance line used to carry the response signals from the sensors back to the indicator circuit 12 in a time multiplexed fashion, for example. The high impedance line 16c is typically shielded to prevent capacitive coupling between the low impedance lines 16a and 16b and high impedance line 16c which could degrade the response signals.
It is well known that a potentially explosive environment like the aircraft fuel tank has to be protected from an excessive transfer of energy over the lines 16 which may result from a variety of potential system faults that may include power fault conditions and lightning strikes, for example, while not impacting the system performance. Generally, protective barrier circuitry 18 is provided in series with the lines 16 to limit the transfer of fault energy from threat side 20 to a safe side 22. The protective barrier 18 limits the fault energy to a level that is below the minimum ignition energy of an explosive mixture within the fuel tank 14. A typical protective barrier used for this purpose is disclosed in the U.S. Pat. No. 6,141,194, issued Oct. 31, 2000, entitled “Aircraft Fuel Tank Protective Barrier and Method”, and assigned to the same assignee as the instant application, which patent being incorporated by reference herein for providing such a protective barrier in greater detail. In the present embodiment, the barrier circuitry 18 is grounded to the ground of the indicator circuit 12 on the unprotected or threat side 20 and grounded to the ground of the tank 14 on the protected or safe side 22. Both grounds are sufficient to carry the currents of any contemplated fault condition.
Since the protective barrier 18 is generally not located directly within a wall of the protected fuel tank 14, some type of wiring 24 is used to couple the excitation signals and response signal at the safe side 22 to the sensors within the fuel tank 14. Accordingly, unless the safe side wiring 24 is configured correctly, it may provide an entry point for unacceptable fault energy levels into the fuel tank regardless of the protective capabilities of the protective barrier 18.
The present invention includes apparatus for and method of configuring the safe side wiring of the protective barrier against unacceptable energy levels that may result from system fault conditions.